1. Field of the Invention
This invention relates to the reduction of the power loss which occurs during the turning on and turning off of semiconductor switches, which are connected in a series with an inductor.
2. Description of the Prior Art
From German published Patent Specification No. 1049962 there is already known a circuit arrangement including at least one semiconductor switch and an inductor connected in series, in which the series combination is connected to a source of direct voltage and in which a stabilized output voltage is provided by way of the said inductance. The switching arrangement concerned also includes a diode connected in parallel with the said inductance and a capacitor connected in series with this inductor. This diode is so poled that it operates in the pass direction only in response to the voltage appearing across the said inductance when the semiconductor switch is turned off. The diode thus serves to prevent damage to the semiconductor switch through the inductive turn-off voltage appearing on the said inductor. That is, the diode acts as an arrestor element for the inductive turn-off voltage appearing on the said inductor. Because of the finite turn-off time of the diode concerned, typically on the order of 0.5 .mu.s, the known circuit arrangement has a relatively high switching power loss. For example, if the rise time dI.sub.1 /dt of the semiconductor is 16A/.mu.s, then for a turn-off time of t.sub.1 = 0.5 .mu.s and a supply direct voltage of U = 400 V a peak power loss P.sub.1 of:
P = dI.sub.1 /dt .multidot. t.sub.1 .multidot. U = 16A/.mu.s .multidot. 0.5 .mu.s .multidot. 400 V = 3.2KW
then occurs in the semiconductor switch.
For a switching frequency f.sub.1 = 30 kHz of the semiconductor switch, and in consequence a periodic time of T.sub.1 = 1/ f.sub.1 = 33 .mu.s, the mean switching loss P.sub.1
of the semiconductor switch concerned then amounts to:
P.sub.1 = dI.sub.1 /dt .multidot. t.sub.1 .multidot. 1/2 .multidot. U .multidot. t.sub. 1 /T.sub. 1 = 16A/.mu.s .multidot. 0.5 .mu.s .multidot. 1/2 .multidot. 400 V .multidot. 0.5 .mu.S/ 33 .mu.s = 24W
thus the known circuit considered operates with a relatively high switching loss.
To reduce the power loss occurring in connection with the switching of a semiconductor switch in a circuit arrangement of the known kind above considered, it is possible to introduce between the switching path of the semiconductor switch and the known series combination, consisting of inductance and capacitor, a parallel combination consisting of a further inductance and the series combination of a resistor and a diode or of a Zener diode and another diode. The diodes in the known series combination are then called on to act in the pass direction only in respect to the voltage appearing on the said further inductance when the semiconductor switch is turned off. When the diode connected in series with the Zener diode mentioned is operating in the forward direction, the Zener diode is operated in the blocked direction. In this case there results a mean switching loss P.sub.1 for the semiconductor switch corresponding to the equation:
P.sub.1 = U/2 .multidot. I.sub.1 /2 .multidot. tv/t.sub. 1 .multidot. tv/T.sub. 1
where I.sub.1 is obtained from the relation I.sub.1 = U/L .multidot. t.sub.1 ; and tv is the measured rise delay time of the supply direct voltage on the inductance in the series combination comprising this and the capacitor. The tv-value is typically of the order of 0.1 .mu.s. When a further inductance having an inductance of L = 30 uH is used, and on the assumption of a supply direct voltage of U = 400 V, as well as a turn-off delay time of t.sub.1 = 0.5 .mu.s for the diode connected in parallel with the said series combination of inductor and capacitor, there is thus obtained in the present case a power loss of: ##EQU1## This value of the switching loss for the semiconductor switch denotes a reduction to 1/60 of the switching loss for the previously known circuit arrangements.
Although the switching loss of the semiconductor switch is reduced to a relatively small value through the above-explained parallel combination between the semiconductor switch and the series combination consisting of inductor and capacitor, the parallel combination concerned itself introduces a substantial loss of power. This power loss is composed of two power loss components. The one power loss component occurs in the turn-on phase of the semiconductor switch; the value of this loss P.sub.1, is given by:
P.sub.1' = 1/2 L (I.sub.1 - I.sub.L1).sup.2.multidot. f.sub.1
where L denotes the inductance of the further inductor, I.sub.1 the current flowing through the semiconductor switch and I.sub.L1 the current flowing through the inductor of the said series combinations.
The other power loss component P.sub.1 " occurs in the turn-off phase of the semiconductor switch. Its value is given by:
P=hd 1"= 1/2 L .multidot. I.sub.L1.sup.2 .multidot. f.sub.1
For example, assuming an inductance L = 30 .mu.H, a peak current I.sub.1 = 6.66A, and a current I.sub.L1 = 4A flowing through the inductance of the said series combination, the sum of the power losses in the parallel combination or in the series combination (diode and resistance or diode and Zener diode) of this parallel combination then amounts to P.sub.s = 10.4W. This value is relatively high.
With the above-mentioned parallel combination consisting of a further inductance and a series combination of diode and Zener or diode and resistor, there also occurs in turning off the semiconductor switch, i.e. on setting the semiconductor switch into the non-conductive condition, an intrinsically undesirable inductive turn-off voltage on the electrode connected with the said parallel combination.
It is a primary object of the present invention to reduce the power losses occurring in connection with the switching of a semiconductor switch in a circuit arrangement of the kind initially described.